Process for treating and recovering pickling waste liquids for stainless steel

ABSTRACT

A process for treating and recovering pickling waste liquids used for the pickling of stainless steel such as nitric-hydrofluoric acid, nitric acid, hydrochloric acid and, sulfuric acid is disclosed, which comprises recovering the nitric-hydrofluoric acid and the iron oxide or metallic ion from the waste liquids of nitric-hydrofluoric acid and nitric acid by two solvent extraction processes and recovering the Cr and Ni containing ferrite from the waste liquids of sulfuric acid or hydrochloric acid by a ferrite formation process. When the waste acid is sulfuric acid, gypsum of a high purity can be obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for treating and recovering apickling waste liquid for stainless steel, particularlynitric-hydrofluoric acid, nitric acid, sulfuric acid or hydrochloricacid waste liquid containing ions of Fe, Cr and Ni used in the picklingof stainless steel sheet.

2. Description of the Prior Art

At the pickling step in the stainless steel making, solution of amineral acid such as nitric-hydrofluoric acid, nitric acid, sulfuricacid, hydrochloric acid or the like is usually used, from which variouspickling waste liquids containing ions of Fe, Cr and Ni are produced.

Heretofore, the treatment of nitric-hydrofluoric acid or nitric acidwaste liquid has exclusively been performed through an alkalineutralizing process. In this case, sludges such as metal hydroxides andmetal fluorides are produced in a large amount, and nitrogen content(NO₃ ⁻) is discharged which is a cause of eutrophication in the publicwater area. Accordingly, it has been demanded to develop a technique forrecovering nitric-hydrofluoric acid and metals from the waste liquid.

In this connection, a solvent extraction process is noted as arecovering technique. This technique is disclosed in Japanese PatentApplication Publication No. 56-42,674, wherein Fe³⁺ is first selectivelyextracted from the waste liquid of nitric-hydrofluoric acid with anorganic solvent containing alkyl phosphoric acid. The thus extractedFe³⁺ is stripped by a process using an aqueous solution of fluorideseries as described in Japanese Patent laid-open No. 57-42,545 andrecovered as an iron oxide or a metallic iron by a process as describedin Japanese Patent laid-open No. 58-15,039 or Japanese PatentApplication Publication No. 58-12,323. On the other hand, at least anequivalent amount of sulfuric acid or hydrochloric acid is added tometal ions contained in the waste liquid after the extraction of Fe, andthen the nitric-hydrofluoric acid is recovered with an organic solventcontaining a neutral phosphate ester. According to this process, thenitric-hydrofluoric acid can be reused in a high yield, but the problemstill remains in the treatment for ions of Ni and Cr included in theresidual liquid after the extraction of nitric-hydrofluoric acid. Forexample, if it is intended to recover Ni and Cr by solvent extractionprocess or the like, the recovering step becomes complicated.

And also, the treatment for sulfuric acid or hydrochloric acid wasteliquid has hitherto been carried out by the alkali neutralizing process.In this case, however, the cost in the oxidation of Fe²⁺ becomesexpensive and a large amount of hydroxide sludge is produced and isdifficult to effectively use. Accordingly, aiming at the presence ofFe²⁺ in the sulfuric acid or hydrochloric acid waste liquid, there hasbeen proposed such a technic that this metal ion is recovered togetherwith ions of Cr and Ni as ferromagnetic oxide by a ferrite formationprocess as shown in Japanses Patent Application Publication No.51-22,307. The thus recovered ferrite is useful for a radio waveabsorber and a vibration damper and has a high additional value.However, this ferrite formation process is inapplicable to the treatmentof nitric acid or nitric-hydrofluoric acid and produces a concentratedsolution of sodium sulfate when being applied to the treatment of thesulfuric acid waste liquid. In the latter case, there is a drawback thatwhen the resulting waste water is led to a treating equipment for usuallime neutralization, a gypsum is produced, resulting in the cause oftroubles.

As mentioned above, the solvent extraction process can treat the nitricacid or nitric-hydrofluoric acid waste liquid, while the ferriteformation process can treat the sulfuric acid or hydrochloric acid wasteliquid. However, it is customary to simultaneously use the nitric acidor nitric-hydrofluoric acid and the sulfuric acid or hydrochloric acidin the pickling step for stainless steel, so that it is desired to treatthese waste liquids synthetically.

SUMMARY OF THE INVENTION

It is an object of the invention to solve the above drawbacks of theprior art, while retaining advantages thereof, and to provide atechnique for treating a pickling waste liquid for stainless steelsynthetically, recovering the nitric-hydrofluoric acid, the includedmetal ions and the like in the effectively reusable form, and preventingthe water pollution.

According to the invention, there is the provision of a process fortreating and recovering nitric-hydrofluoric acid, nitric acid andhydrochloric acid waste liquids used for the pickling of stainlesssteel, which comprises removing Fe³⁺ from the nitric-hydrofluoric acidand nitric acid waste liquids by a solvent extraction process to recoverit as an iron oxide or a metallic iron, recovering the nitric acid andthe hydrofluoric acid by another solvent extraction process, convertingthe remaining ions of Cr and Ni into hydroxides by adding an alkali, andrecovering them as ferromagnetic oxide by a ferrite formation processusing the hydrochloric acid waste liquid as an acid for resolving thehydroxide and an iron source of Fe²⁺.

According to the invention, there is also the provision of a process fortreating and recovering nitric-hydrofluoric acid, nitric acid andsulfuric acid waste liquids used for the pickling of stainless steel,which comprises removing Fe³⁺ from the nitric-hydrofluoric acid andnitric acid waste liquids by a solvent extraction process to recover itas an iron oxide or a metallic iron, recovering the nitric acid and thehydrofluoric acid by another solvent extraction process, converting theremaining ions of Cr and Ni into hydroxides by adding an alkali,recovering them as ferromagnetic oxide by a ferrite formation processusing the sulfuric acid waste liquid as an acid for resolving thehydroxides and an ion source of Fe²⁺, and recovering as a gypsum asulfate group included in the filtrate after the ferrite process.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flow diagram illustrating an embodiment of the process fortreating and recovering pickling waste liquids for stainless steelaccording to the invention, which is composed of a solvent extractionstep 100 and a ferrite forming step 200.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a flow diagram illustrating one embodiment of the procedure ofthe present invention, which is divided into a solvent extraction step100 and a ferrite forming step 200 but constitutes one flow as a whole.

The nitric-hydrofluoric acid waste liquid Mo and the nitric acid wasteliquid No are mainly treated at a solvent extraction step. Mo or amixture of Mo and No mainly contains Fe³⁺ in the form of FeF₂ ⁺, fromwhich the iron is removed at 1 by an ion exchange reaction with anorganic solvent A containing an alkyl phosphoric acid (hereinafterabbreviated as HR) as an extractant. A typical example of the alkylphosphoric acid is di-(2-ethylhexyl)phosphoric acid (which isabbreviated as D2EHPA).

    FeF.sub.2.sup.+ +3HR→FeR.sub.3 +H.sup.+ +2HF        (1)

    FeF.sub.2.sup.+ +Hr→FeF.sub.2 R+H.sup.+             (2)

The solvent A containing the extracted Fe³⁺ is reacted at 2 with astripping solution D consisting mainly of NH₄ HF₂ to form a crystal X of(NH₄)₃ FeF₆. ##STR1##

A part of the extractant after the stripping of the iron is in anammonia form (NH₄ R) as shown in the equation (5), so that it isconverted by contacting with an aqueous phase containing hydrochloricacid at 3. The solvent A containing the extractant converted into ahydrogen form (HR) is again used for the extraction of iron at 1.

The stripping solution containing the crystal X is subjected to aliquid-solid separation at 4, and then the crystal X is decomposed intoan iron oxide H or a metallic iron I in air or a hydrogen stream at 5.

    (NH.sub.4).sub.3 FeF.sub.6 +3/4.sub.2 →3NH.sub.4 F+3/2F.sub.2 +1/2Fe.sub.2 O.sub.3                                      (6)

    (NH.sub.4).sub.3 FeF.sub.6 +3/2H.sub.2 →3NH.sub.4 F+3HF+Fe(7)

The decomposition gas E produced at 5 contains NH₄ F, HF and the like,so that it is absorbed and reused as the stripping solution D. Thecrystal X formed by the reaction at 2 has a high purity as (NH₄)₃ FeF₆,so that the resulting product H or I has also a high purity.

On the other hand, a part of the waste acid after the extraction of theiron is returned to a nitric-hydrofluoric acid pickling tank as aniron-free acid M₁ for reuse, while the remainder is led to a system forrecovering nitric-hydrofluoric acid, the ratio of which is determined sothat the amounts of Cr and Ni included therein may be balanced with thedissolving amounts of Cr and Ni in the nitric-hydrofluoric acid picklingtank. First of all, the metal nitrates are converted into chlorides bythe addition of hydrochloric acid C at 6, which is then contacted withan organic solvent B containing a neutral phosphate ester as anextractant at 7 to extract the nitric acid and the hydrofluoric acid.Tributyl phosphate (TBP) is a typical example of the neutral phosphateester.

    HNO.sub.3 +TBP⃡HNO.sub.3 ·TBP         (8)

    HF+TBP⃡HF·TBP                         (9)

The nitric acid and hydrofluoric acid extracted in the solvent B arestripped at 8 with water W to recover a mixed acid M₂, which is reusedin the nitric-hydrofluoric acid pickling tank.

The aqueous phase (raffinate) after the extraction ofnitric-hydrofluoric acid at 7 contains hydrochloric acid, so that it isused for the conversion of the solvent A (NH₄ R→HR) at 3 as mentionedabove and as a result contains NH₄ ⁺. Now, such an aqueous phase isneutralized at 9 with aqueous solutions of calcium chloride J and sodiumhydroxide K to obtain a hydroxide slurry L composed mainly of Cr and Ni.The supernatant liquid is fed to a waste water treating equipment Tafter the recovery of ammonia water P at 10. The ammonia water P isreused for adjusting the concentration of the stripping solution and thelike.

The above procedures are included in the solvent extraction step 100.

The hydroxide slurry L produced at 9 and composed mainly of Cr and Ni isfed to a subsequent ferrite formation step 200, at where it is treatedtogether with the sulfuric acid waste liquid So or the hydrochloric acidwaste liquid Co. L and So or Co are mixed with an aqueous solution K ofsodium hydroxide at 11, whereby the hydroxides of Cr and Ni are resolvedby the acid content and neutralized together with Fe²⁺ to form acomposite hydroxide. An embodiment of the reaction equation is shown asfollows.

    (3-y-z)Fe.sup.2+ +yCr.sup.3+ +zNi.sup.2+ +(6+y)OH.sup.- →Ni.sub.z Fe.sub.3-y-z Cr.sub.y (OH).sub.6+y                        (10)

wherein 0≦y<2 and 0≦z≦1. Then, the aqueous solution K of sodiumhydroxide is added at a liquid temperature of 60°-80° C. at 12 and air Qis blown thereinto while retaining the pH value of 9-11, whereby a partof Fe²⁺ in the composite hydroxide is oxidized into Fe³⁺ to form aferromagnetic spinel compound (ferrite).

    Ni.sub.z Fe.sub.3-y-z Cr.sub.6 (OH).sub.6+y +(1/2-y/4)O.sub.2 →Ni.sub.z Fe.sub.3-y-z Cr.sub.y O.sub.4 +(3+y/2)H.sub.2 O(11)

This reaction liquid is treated by settling and magnetic separation at13 to obtain a ferrite F. The supernatant liquid is fed to the wastewater treating equipment T when the waste acid is the hydrochloric acidwaste liquid Co. While, when it is the sulfuric acid waste liquid So,the supernatant liquid contains SO₄ ²⁻, so that it is added with calciumchloride J at 14 to form a gypsum G and then the gypsum G is separatedat 15, after which the resulting filtrate is fed to the waste watertreating equipment T.

    Na.sub.2 SO.sub.4 +CaCl.sub.2 +2H.sub.2 O→CaSO.sub.4 ·2H.sub.2 O+2NaCl                                (12)

The resulting gypsum G does not substantially contain any metal besidesCa, so that the whiteness is high.

As explained above in detail, the feature of the invention lie in thesynthetic treatment and the effective recovery of various waste acidsproduced in the pickling step for stainless steel by using two solventextraction processes and a ferrite formation process. That is, thenitric-hydrofluoric acid and the iron oxide or metallic iron of a highpurity can be recovered from the nitric-hydrofluoric acid and nitricacid waste liquids, while the Cr and Ni containing ferrite can beobtained from the sulfuric acid and hydrochloric acid waste liquids andparticularly gypsum of a high purity can be obtained in case of thesulfuric acid waste liquid. Additionally, the invention takes care ofprevention of the water pollution due to the discharge of nitrogencontent (NO₃ ⁻) and fluorine content (F⁻) and thus can be said to be anexcellent waste acid treating technique in environmental control.

The following examples are given for the purpose of illustration of theinvention and are not intended as limitations thereof.

EXAMPLE 1

A waste liquid was treated at a rate of 1 m³ /hr in a recovery equipmenthaving a treatment capacity for nitric-hydrofluoric acid waste liquid of1 m³ /hr. The composition of the waste liquid was as follows:

Fe³⁺ =39.0 g/l,

Cr³⁺ =9.9 g/l,

Ni²⁺ =4.5 g/l,

HNO₃ =180 g/l, and

HF=44 g/l.

This waste liquid was subjected to an iron extraction with an organicsolvent A consisting of 30 v/v% of D2EHPA and 70 v/v% of n-paraffin at aflow rate of aqueous phase to organic phase of O/A=2.2 by means of afour-step mixer-settlers. The concentration of Fe³⁺ in the organic phaseafter the extraction of iron was 17.6 g/l and the concentrations of Fe³⁺and HF in the aqueous phase were 0.02 g/l (the extraction rate of Fe³⁺was 99.9%) and 35 g/l, respectively.

The organic solvent A containing 17.6 g/l of the extracted Fe³⁺ was fedto a crystallizer having a crystal producing capability of 45 t/month,at where it was mixed with a solution of 125 g/l of NH₄ HF₂ at a flowrate O/A=1/1.75 to strip Fe³⁺ and to obtain a white crystal of (NH₄)₃FeF₆. This crystal was fired at 450°-550° C. in a rotary kiln having aproduction capability of iron oxide of 15 t/month. The resulting ironoxide contained 0.30% of phosphorus and 10.0% of fluorine. By refiringthe iron oxide at 800° C., the phosphorus and fluorine contents werereduced to 0.21% and not more than 0.02%, respectively, and the purityas Fe₂ O₃ was not less than 99.5%.

After the iron extraction, nitric-hydrofluoric acid was extracted at arate of 0.3 m³ /hr from the waste acid (Fe³⁺ =0.02 g/l, Cr³⁺ =9.9 g/l,Ni²⁺ =4.5 g/l, HNO₃ =180 g/l, HF=35 g/l). The organic solvent B wascomposed of 70 v/v% of TBP and 30 v/v% of n-paraffin. The extraction ofnitric-hydrofluoric acid at O/A=3 and the stripping thereof at O/A=2.7were carried out to obtain 153 g/l of HNO₃ (recovery 93.5%) and 25 g/lof HF (recovery 78.6%) as a recovered acid.

The aqueous phase (raffinate) after the extraction ofnitric-hydrofluoric acid was neutralized with solutions of 35% CaCl₂ and24% NaOH to obtain a hydroxide slurry having a water content of 78%.

EXAMPLE 2

The hydroxide slurry obtained in Example 1 was dried to obtain a cakehaving Cr content of 23.2% and Ni content of 10.6%. 25 g of the cake wascharged into a four-necked round flask of 1 l capacity, to which 500 mlof the sulfuric acid waste liquid for stainless steel (H₂ SO₄ =310 g/l,Fe²⁺ =54 g/l, Cr³⁺ =9 g/l, Ni²⁺ =1 g/l) was added and stirred withheating at 70° C., whereby the hydroxide was dissolved. Then, theresulting product was neutralized with a solution of 24% NaOH and thenair was blown at a rate of 2 l/min for 3.5 hours, while holding a pHvalue of 10.8 at 70° C. This reaction liquid was filtrated to obtain ablack powder having a saturation magnetization of 39 emu/g. It is clearthat this powder is a ferromagnetic oxide of spinel type from the resultof a X-ray diffraction.

The above filtrate was colorless and transparent, to which was added asolution of 20% CaCl₂ to obtain a purely white gypsum having a watercontent of 6.2%.

What is claimed is:
 1. A process for treating and recoveringnitric-hydrofluoric acid, nitric acid and hydrochloric acid wasteliquids used for the pickling of stainless steel, which comprisesremoving Fe³⁺ from the nitric-hydrofluoric acid and nitric acid wasteliquids by a solvent extraction process to recover it as an iron oxideor a metallic iron, recovering the nitric acid and the hydrofluoric acidby another solvent extraction process, converting the remaining ions ofCr and Ni into a hydroxide by adding an alkali, and recovering them asferromagnetic oxides by a ferrite formation process using thehydrochloric acid waste liquid as an acid for resolving the hydroxideand an ion source of Fe²⁺.
 2. A process for treating and recoveringnitric-hydrofluoric acid, nitric acid and sulfuric acid waste liquidsused for the pickling of stainless steel, which comprises removing Fe³⁺from the nitric-hydrofluoric acid and nitric acid waste liquids by asolvent extraction process to recover it as an iron oxide or a metalliciron, recovering the nitric acid and the hydrofluoric acid by anothersolvent extraction process, converting the remaining ions of Cr and Niinto a hydroxide by adding an alkali, recovering them as ferromagneticoxides by a ferrite formation process using the sulfuric acid wasteliquid as an acid for resolving the hydroxide and an ion source of Fe²⁺,and recovering as a gypsum a sulfate group included in the filtrateafter the treatment for the ferrite formation process.